1. Field of the Invention
This invention relates to semiconductor nanocrystal probes for biological applications wherein the probes include a plurality of semiconductor nanocrystals capable of providing a detectable signal in response to exposure to energy.
2. Description of the Related Art
Fluorescent labeling of biological systems is a well known analytical tool used in modern biotechnology as well as analytical chemistry. Applications for such fluorescent labeling include technologies such as medical (and non-medical) fluorescence microscopy, histology, flow cytometry, fluorescence in-situ hybridization (medical assays and research), DNA sequencing, immuno-assays, binding assays, separation, etc.
Conventionally, such fluorescent labeling involves the use of an organic dye molecule bonded to a moiety which, in turn, selectively bonds to a particular biological system, the presence of which is then identified by excitation of the dye molecule to cause it to fluoresce. There are a number of problems with such an analytical system. In the first place, the emission of light of visible wavelengths from an excited dye molecule usually is characterized by the presence of a broad emission spectrum as well as a broad tail of emissions on the red side of the spectrum, i.e., the entire emission spectrum is rather broad. As a result, there is a severe limitation on the number of different color organic dye molecules which may be utilized simultaneously or sequentially in an analysis since it is difficult to either simultaneously or even non-simultaneously detect or discriminate between the presence of a number of different detectable substances due to the broad spectrum emissions and emission tails of the labeling molecules. Another problem is that most dye molecules have a relatively narrow absorption spectrum, thus requiring either multiple excitation beams used either in tandem or sequentially for multiple wavelength probes, or else a broad spectrum excitation source which is sequentially used with different filters for sequential excitation of a series of probes respectively excited at different wavelengths.
Another problem frequently encountered with existing dye molecule labels is that of photostability. Available fluorescent molecules bleach, or irreversibly cease to emit light, under repeated excitation (104-108 cycles of absorption/emission). These problems are often surmounted by minimizing the amount of time that the sample is exposed to light, and by removing oxygen and/or other radical species from the sample.
In addition, the probe tools used for the study of systems by electron microscopy techniques are completely different from the probes used for study by fluorescence. Thus, it is not possible to label a material with a single type of probe for both electron microscopy and for fluorescence.
It would, therefore, be desirable to provide a stable probe material for biological applications preferably having a wide absorption band and capable of providing a detectable signal in response to exposure to energy, without the presence of the large red emission tails characteristic of dye molecules (thereby permitting the simultaneous use of a number of such probe materials, each, for example, emitting light of a different narrow wavelength band) and/or capable of scattering or diffracting radiation. It would also be equally desirable to provide a single, stable probe material which can be used to image the same sample by both light and electron microscopy.